In a half-bridge type circuit or a full-bridge type circuit, an arm formed by connecting two semiconductor switching elements in serial is used. A means which applies a rectangular-wave shaped voltage or a trapezoidal-wave shaped voltage to the respective elements is employed to drive the elements.
FIG. 10 is a diagram showing an example of such a driving circuit a.
In the figure, FETs (field effect transistors) serving as switching elements S1, S2 are connected in series on the secondary side of a pulse transformer T. The gate of the FET used as the switching element S1 is connected to the start end side of a secondary winding T2h and the drain thereof is connected to a DC power supply terminal b of a predetermined voltage. The gate of the FET used as the switching element S2 is connected to the termination end side of a secondary winding T2l and the source thereof is grounded. The drain of this FET is connected to the source of the former FET (S1)
A DC power supply “E”, represented by the symbol for a constant voltage supply, is connected to a coupling point between primary windings T1h, T1l of the pulse transformer T so as to supply a predetermined voltage to these primary transformers. An FET (which constitutes a switching element S3) is connected to the primary winding T1h through a resistor R1, whereas an FET (which constitutes a switching element S4) is connected to the primary winding T1l through a resistor R2. Each of these FETs is configured in a grounded-source type and the drains of these FETs are connected to the corresponding primary windings through the corresponding resistors, respectively.
FIG. 11 is a graph showing the temporal changes (i.e., as a function of time “t”) of a voltage “Vgs(t)” (shown as a dotted line) between the source and the gate of the FET used as the switching element S1, and a current “I(t)” flowing through the resistor R1 assuming that the switching element S1 is switched by driving the switching element S3.
In the case where the switching element is switched by using a rectangular-wave shaped voltage or a trapezoidal-wave shaped voltage, it is necessary to supply electric charges quickly to the gate capacitance so as to perform the charging and discharging operation, whereby the current I (t) exhibits a steep pulse shape. However, when the charge and discharge speed is too fast, the peak value of the current becomes too high, which may cause breakdown of the FET. Thus, in this example, the resistors R1 and R2 are used as limiting elements for suppressing the peak value of the current flowing during a charging and discharging operation.
In the foregoing circuit configuration, a problem may occur if a measure for reducing the power loss is not performed adequately.
For example, in the circuit of FIG. 10, when switching the elements S1 and S2 at a high frequency by controlling the driving operations of the elements S3 and S4, the power loss of the resistors R1 and R2 increases and hence a problem arises that the electric efficiency of the entire circuit degrades.
It is possible to increase the electric efficiency by regenerating energy consumed in the current limiting resistors on the power supply side. However, in this case, when the regeneration is accompanied by a complicated circuit configuration and control method, problems arise in connection with cost reduction, reduction of the number of parts, etc.